Summary: SMPS design and analysis requires an understanding of many different areas of study. This article is intended to help the reader become familiar with the several different SMPS topologies, and their general performance characteristics. The major information is contained in the referenced articles.

SMPS types:

Dependent on who does the classification, several different switching converter types may be enumerated. These may include the familiar Buck, Boost, Buck-Boost, Cuuk, forward converter and flyback converter. Also listed are full-bridge, half bridge, ferro-resonant converters, as well a power factor correction converters and perhaps even a few others!

The references provide descriptions and operational information on many of these circuits as well as the more familiar ones. It is expected that the interested person will have downloaded and read those documents and is familiar with the operation and characteristics of these converters.

Analysis problems:

Modern SMPS designs are feedback control systems. One might think at first that familiar stability analysis tools could be used; however this is not the case. The typical SPICE stability analysis centers about determination of the AC phase - gain characteristics. However, SPICE calculates an AC small signal model about the DC operating point of a network. Where is the operating point of a switching transistor varying between hard on and hard off?

SMPS designs represent an extreme analysis problem. At one level, the converter is switching at frequencies reaching into the hundreds of kilohertz, however, its performance requirements are dependent on feedback shaping circuits and load filtering with long time constants. Often much analysis time is required to reach the steady state conditions. Having reached these steady state conditions, if now the load is step changed, or other conditions occur, another long simulation interval is required.

To this is the added problem of simulation of the PWM controller chips used with most SMPS designs. This could involve thousands of transistors and devices, many of which are operating as digital devices, creating numerous state changes, causing computations to slow at each transition. Analysis can be extremely time consuming. To answer the analysis problem, as well as addressing stability of designs, a combination of three general methodologies are used.

The first is the classical 'exact' method. This model provided more exact information regarding circuit performance at start up, response to step loads and other changing load conditions, audio susceptibility, EMI, and losses at the cost of long computational times. However stability cannot be examined in this method.

A second method is to replace the transistor switch and/or the PWM controller chip with an idealized switching device. This offered many advantages; however, the problem of stability analysis remains.

The third method involves averaging the effects during a switch cycle. As an example, for any topology we can model the effects on the output while the switch is 'on' and while it is off, during a single switching cycle. Now if these effects are time averaged, and the model produced instead of a switched waveform this average effect (or a signal which produced this average effect) then an 'average' operating point could be determined and a stability analysis performed. This is in fact an 'ideal transformer' application where a DC input voltage is transformed by a variable (duty-cycle) controlled transformer.

Now this presumes that the switching frequency is considerably higher than the cut-off of the output filter and other frequency sensitive elements, which is almost always true.

Practically the first method is infrequently used as such for controller chip manufacturers seldom provide detailed SPICE models. What one does find is they will provide simplified block diagrams of their chips. Typically one models the controller chip using XSPICE devices or logical functions (to minimize the computational burden) and uses this chip model with an actual switching device or, as in the second method with an idealized switching device.

The XSPICE PWM chip model can also be modified to produce a DC level proportional to the (error signal commanded) percent modulation of the switching device, producing a controlling level to an ideal variable transformer, which passes DC and is the penultimate DC - DC converter.

Classes of models:

From the preceding two general classes of practical simulation models result. These are the switched models and the averaged models.

Switched models vary in type and complexity. Some use (nearly) ideal switches and diodes to represent SMPS switching behavior while others idealize mainly the PWM controller chip, although portions of the feedback loop may also be idealized. The object here is to maintain a switching cycle model of the converter, although depending on the model and particularly the switching element, losses may be neglected. These models are suitable for use in transient analysis. In general they all neglect something, as all models of real devices must, but they are in general suited for investigation of transient SMPS behavior at both input and output terminals.

Averaged models are best suited for investigation of SMPS stability in AC analysis, as well as audio susceptibility; however their behavior under transient loading conditions as well as load levels is often close to observed behavior.

There are a large variety of models of each class. Different models are present for forward and flyback converters, buck and boost converters. And for each type, several people have developed models that have been identified with their name, such as Ridley, Ben-Yaakov and Basso models. Although NOT identified with a particular model, Dr Vincent Bello is often accorded the title as the 'father' of the averaged mode SMPS simulation.

As compared to 'exact' model representations, these models will simulate rather rapidly and produce results that closely represent actual observed behavior, and is some cases such as AC characteristics, stability which cannot otherwise be modeled.

A listing of some of the many models is present in an announcement by Spectrum Software in their Winter 2000 newsletter. These models are described in the book by Christophe Basso in his book, SMPS Power Supply Cookbook.

No attempt will be made here to describe the models or how they were derived, however some specific SMPS topologies will be discussed in later papers.

Conversion Methodologies:

There are two general methods of producing the desired output levels. The first is forward conversion, and the second is flyback mode. Each has advantages and disadvantages.

The first method uses power transformation to achieve the desired results. During the switch 'ON' time power is delivered to the load. The second stores energy during the switch conduction interval, which will be delivered when the switch is not conducting. The referenced articles describe each method.

There is even a method that delivers energy to the load during both the switch on and off intervals, producing a smooth power draw from the source instead of the typical pulsating loading. This has the effect of minimizing conducted interference at the source.

Control Methods:

One would be remiss not to mention that there are two general classes of SMPS control, namely voltage mode and current mode. Voltage mode consists of varying the switching pulse width in response to an error voltage level. This is simple, however, the supply is not inherently protected against short circuits at the output.

Current mode control uses a combination of an error voltage level AND a measure of output current to arrive at modified control of the duty cycle. More specifically, the output current is limited in magnitude causing the output current to be limited under short circuit conditions. There are more considerations than just this, which are covered in the references.

Operational Modes:

There are two general modes of operation, continuous and discontinuous. In the continuous mode, current through the filter inductor (as in a buck or boost realization) never reaches zero, and discontinuous where the current is deliberately allowed to reach zero, or 'dry out'. Discontinuous operation causes problems in modeling, as the average model has to be adjusted to allow for the third state where the output capacitor alone supplies load current. There are however advantages to discontinuous mode operation, which are described in the literature and hopefully will be covered in subsequent parts.

Many models are suitable only for the continuous mode of operation. Amazingly, ingenious yet simple models have been developed which will accommodate either and both modes of operation.

Conclusions:

It is not a trivial exercise to design and model an SMPS, as there are many subtle considerations. As one example, one can design a system that will indeed produce the desired results, however, after the design is completed, often one must consider conducted EMI at the supply mains. The inclusion of filtering can cause an otherwise stable SMPS design to become unstable.

Another pitfall could be the effects of catastrophic unloading of the converter. If the output L-C filtering is not properly chosen, the output could rise to dangerous levels destroying attached IC devices. This can occur in a single switching cycle, before the supply can respond.

However it is NOT a hopeless or impossible task to properly model operating SMPS implementations, and there are sufficient tools to enable this to be done using B2SPICE. The problems lie in the fact that most of the models are tailored to other SPICE implementations, as are the published books and articles, seemingly advertisements for those products. Subsequent parts of this discussion will examine some existing models, and provide a means of analysis of SMPS designs with B2Spice, or for that matter, any full SPICE3 simulator.

References:

1. Just Battery Links.com. This site has many interesting links to SMPS articles.
http://www.just-battery-links.com/design_flyback_power_supply.html

2. Electronics home page. Many interesting ideas here.
http://www.hills2.u-net.com/electron/smps.htm#buck

3. Power Designers home page.
http://www.hills2.u-net.com/electron/smps.htm#buck

4. Chris Basso Switch Mode Power Supply Cookbook
A source for many SMPS models (which need varying amounts of work for B2SPICE usage. http://perso.wanadoo.fr/cbasso/Spice.htm#Links

5. TI Application Note SLUA059A, Understanding the Power Stages in Switch Mode Power Supplies.
http://focus.ti.com/lit/an/slva059a/slva059a.pdf